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Full text of "Handbook Of Chemical Engineering - I"

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PYROMETRY                                         421
For extreme precision it is always necessary to apply such corrections, but for most industrial processes, thermocouples which are sufficiently reproducible can be secured, so that the corrections are negligibly small and may be omitted. Thus the temperature-electromotive force relations of different homogeneous and uncontaminated chromel- alumel couples should not vary in general by more than 5 or 10C. and of platinum-platinum rhodium couples by more than 2 or 3C. There are few industrial processes carried out at high temperatures under such accurate temperature control that the variations in the calibration of the different new platinum-platinum rhodium thermocouples of the same type warrant consideration. The variations in the calibration of different base-metal couples are frequently corrected by use of series or shunt resistance, but most methods so far devised are rather unsatisfactory and some of the compensation methods after continued use may develop larger errors than those arising from the irreproducibility of the couple. The above statements in regard to reproductibility of different couples of the same type apply only for new couples. After a couple has been used for some tune, especially a base-metal couple, or has become contaminated in any manner, the calibration may change considerably. All thermocouples should be checked frequently, as installed, to determine whether or not the calibration has altered.
Mounted Thermocouples.The question of properly mounting and protecting a thermocouple is of great importance. The type of protection necessary depends upon the particular industrial process for which a couple is employed. In the laboratory if it be desired to measure the temperature of a perfectly clean platinum-wound porcelain furnace containing no material such as iron, etc. which could contaminate platinum, a rare-metal couple may be used without any protection whatever. For most laboratory experiments, however, a rare-metal couple requires protection. The usual rare-metal couple consists of wires 0.5 mm. or preferably 0.6 mm. in diameter and from 50 to 125 cm. in length. Wires as small as 0.1 mm. and even less are frequently used for special research work. One or both of the wires are insulated by threading them through small porcelain or quartz tubes.
In measuring temperatures below about 1,400C., two-hole porcelain tubes are very useful for insulating both wires, but for higher temperatures separate tubes should be used. The hot junction of the couple is made by fusing the two wires in an arc or oxygen-illuminating gas flame. The couple and insulating tubes are inserted in a small outside protecting tube of porcelain glazed on the outside only, or of fused silica, hemispherically closed at one end. On the open end of the protecting tube may be mounted the head of the couple which serves as a handle and as a support for rigidly holding the wires of the couple. The couple wires frequently extend beyond the head of the couple so that their ends may be maintained at some controlled cold junction temperature. Usually the wires terminate in binding posts on the couple head, in which case the cold-junction temperature may be controlled by water jackets, or may be allowed to remain that of the surroundings, or the couple may be fitted with one of the various devices discussed later for the elimination of cold-junction errors.
Base-metal couples for laboratory use may be constructed in much the same manner described above and may be made of wire as small as No. 20 or of very much smaller wire for certain types of research at lower temperatures. Small wires, however, are readily oxidized completely at high temperatures so that for continuous operation in industrial installations the couples are constructed of No. 8 or No. 6 wire or of still larger wire when there is danger of contamination. The hot junction is fused as in the case of rare-metal couples, and usually the two wires are twisted